1. Field of the Invention
The present invention relates to semiconductor devices and fabrication thereof. More particularly, the present invention relates to a method of effectively and uniformly etching refractory metals, refractory metal alloys and refractory metal silicides for use in semiconductor devices.
2. Discussion of the Background
In semiconductor fabrication, devices may be formed on a semiconductor wafer or substrate, which is typically made of silicon. Above the wafer, there may be disposed a plurality of layers from which the devices may be fabricated.
To form the devices, a portion of the layers is patterned using a suitable etching technique and an appropriate etchant. Semiconductor processing makes extensive use of etching for active area definition, gate recesses, waveguide formation and so on.
Refractory metals such as, for example, molybdenum (Mo), titanium (Ti) and tungsten (W), along with their alloys and silicides, may be used in manufacturing various semiconductor devices. As used herein, the term “refractory metal-containing material” refers to a material containing a refractory metal, a refractory metal alloy or a refractory metal silicide. As used herein, the term “refractory metal-containing layer” refers to a layer of a semiconductor device that contains a refractory metal, a refractory metal alloy or a refractory metal silicide.
Conventional technology for etching a layer containing a refractory metal-containing material uses SF6 or other fluorine-containing compound such as CF4 as the etchant, along with BCl3 and possibly CF4, Cl2 and O2. The process is operated at a high source (or TCP) power, typically about 500 to about 600 watts, and a low bias (or RF) power, typically about 70 to about 150 watts.
For example, U.S. Pat. No. 4,923,562 to Jucha et al. discloses an apparatus and a method for anisotropically etching refractory metals using a feed gas mixture that includes a fluorine source, a bromine source and an oxygen source.
Similarly, U.S. Pat. No. 5,853,602 to Shoji discloses a method of dry etching for patterning a refractory metal layer using a gaseous mixture of SF6/Cl2/CO as the etching gas. The SF6 and Cl2 supply fluorine and chlorine radicals to etch the refractory metal layer, while the CO produces a reaction product that is deposited on the side surface of the refractory metal layer and prevents the fluorine and chlorine radicals from etching the sides of the refractory metal layer.
U.S. Pat. No. 5,143,866 to Matsutani discloses a dry etching method for refractory metals and their compounds using a mixed gas composition of an etchant gas for etching the refractory metal and a deposit gas for depositing the refractory metal such that the deposited refractory metal protects the side walls of the refractory metal to be etched from side etching. The deposit gas is a halide of the refractory metal that is to be etched.
There are problems, however, with the conventional technology. One problem is that the etch rate across the wafer is not uniform. The differential in etch rate uniformity in the conventional process is typically about 20 to 30 percent. Such a large differential in etch rate uniformity requires a large amount of overetch time in order to completely clear or etch away the refractory metal-containing layer within the etching area.
Another problem with the conventional process is poor oxide selectivity. Selectivity refers generally to the ability of an etchant source gas to discriminate between the different layers of the semiconductor device that may be exposed during an etch. Here, the oxide selectivity is expressed as the ratio of the etch rate of the refractory metal-containing material to the etch rate of the oxide. For a given etch, an etchant source gas having a low oxide selectivity tends to etch away at the oxide layer at a higher rate than an etchant source gas having a high oxide selectivity. The poor oxide selectivity, which here is about 1:1 or less, causes an oxide gouge as a result of the overetch step in areas where the refractory metal-containing layer has already been completely etched away.
Yet another problem is that the existing technology cannot meet the requirement of a residual oxide layer of no less than about 200 Angstroms in a M-I-M capacitor. A residual oxide layer of at least no less than about 200 Angstroms ensures reliability of the etched capacitor. The combination of the non-uniform etch rate and the poor oxide selectivity means that the existing technology is not capable of satisfactorily completing the etch (i.e., removing all the refractory metal-containing material and leaving a minimum of no less than about 200 Angstroms of residual oxide in the etched areas).
Yet another problem with the conventional process is that detection of the end point of the etch is not reliable.
In view of the aforementioned deficiencies attendant with the prior art methods, it is clear that there exists a need for a method of etching refractory metals, refractory metal alloys and refractory metal silicides having a uniform etch rate and an increased selectivity to oxide.